Water is an unavoidable contaminant in fuel. It can affect fuel system component reliability and lead to operational delays and increased maintenance activities. In addition, the propensity for microbiological contamination is directly proportional to the presence of water and the temperature within fuel tanks. Sources of water in aircraft fuel tanks is from fuel loaded into the aircraft fuel tanks during refuel (dissolved water) and from air entering the aircraft fuel tanks via its ventilation system. It is estimated that up to 30-40% of water in the fuel of aircraft fuel tanks is currently entering via the ventilation system, depending on atmospheric conditions.
During refill while the aircraft is on the ground, fuel is loaded onto the aircraft at a high volume flow rate to reduce the turnaround time. The fuel displaces the air in the fuel tanks as the tanks fill up. The air is pushed out of the fuel tanks via the ventilation system.
During climb, due to decreasing ambient pressure as the aircraft ascends, air expands in the ullage. Although the engines are consuming fuel, the rate of fuel consumption is less than the rate of air expansion. This results a net outflow of air through the ventilation system.
During cruise, the fuel level decreases steadily as the engines consume the fuel. A decrease in the fuel level causes an increase in the ullage volume, and excess air is drawn in from ambient via the ventilation system to equalise pressures. At cruise, the ambient air is relatively cold and dry.
During descent, due to increasing ambient pressure as the aircraft descends, air contracts in the ullage. This results a net inflow of ambient air through the ventilation system. The ingress of ambient air brings relatively warm, humid air into the fuel system. Water condenses when the humid air comes into contact with cold structures and surfaces in the fuel system. The cold surfaces may be, for example, the fuel tank and equipment surfaces, or the fuel-air interface. The fuel system is cold during descent as the aircraft will likely have just spent some time at cold, cruise temperatures.
A desiccative dehumidifying device can be used in an aircraft fuel tank ventilation system to dehumidify the ingress humid air. However, to reduce maintenance requirements there is a need for a system and method for regenerating (drying) the desiccant medium of the dehumidifying device.